An extruder or a continuous mixer includes a barrel, the barrel having an axially long cavity portion (chamber), and a kneading screw inserted into the interior of the barrel. The kneading screw includes a feed section for feeding a supplied material from an upstream side to a downstream side and a kneading section for imparting a shear force to the material to effect kneading, the feed section and the kneading section being axially disposed side by side. In the kneading section, an additive, e.g., filler, and a synthetic resin different from a matrix resin composed of thermoplastic resin are added to the matrix resin and kneading is performed to mix them with high dispersibility.
How long the material stays in the kneading section exerts a great influence on the degree of kneading of the material. For example, the longer the retention of the material in the kneading section, the larger the number of times of kneading and the higher the degree of material kneading. Conversely, if the material passes through the kneading section quickly, the number of times of kneading decreases and so does the degree of kneading. That is, the degree of material kneading can be enhanced by allowing the material to stay longer in the kneading section.
For the above purpose, in a conventional extruder or continuous mixer there is provided a kneading degree adjusting mechanism for damming up the flow of material on a downstream side of a kneading section, allowing the material to stay in the kneading section. This kneading degree adjusting mechanism adopts the types shown in (1) to (3) below.
(1) Gate Type Kneading Degree Adjusting Mechanism
In each of Patent Documents 1 to 12 there is disclosed a gate type kneading degree adjusting mechanism having gate members for damming up the flow of material. The gate members include a pair of plate-like members installed in an inner periphery of a barrel. From an inner wall of the barrel the gate members can protrude and retract to and from a chamber side formed in the interior of the barrel. When the gate members are protruded into the chamber, the material is dammed up and stays in the kneading section. As a result, the material kneading degree becomes high.
(2) Slot Bar Type Kneading Degree Adjusting Mechanism
In each of Patent Documents 13 to 16 there is disclosed a slot bar type kneading degree adjusting mechanism having gate bars for damming up the flow of material. The gate bars are each rotatable about an axis perpendicular to a barrel and cross the barrel so as to graze a part of an outer periphery surface of the barrel. The portion of each gate bar where the gate bar crosses the barrel is cut out so as to be along an inner periphery surface of the barrel in a fully open condition of a material channel. As the gate bar is rotated about its axis, a part of the gate bar approaches an outer periphery surface of a cylindrical portion of a kneading screw, whereby the gap between the barrel and the kneading screw is narrowed. As a result, the material stays in the kneading section and the material kneading degree becomes high.
The gate type and slot bar (gate bar) type kneading degree adjusting mechanisms are typical as kneading degree adjusting mechanisms. In each of Patent Documents 17 to 19 there is described a kneading degree adjusting mechanism of a type similar to the gate type or the slot bar type.
(3) Kneading Degree Adjusting Mechanisms of Other Types
In each of Patent Documents 20 to 23 there is described a kneading degree adjusting mechanism having on a downstream side of a kneading section a reduced-diameter section with a diameter smaller than that of the kneading section. In the reduced-diameter section it is possible to adjust the degree of narrowing of a material flow. That is, in the kneading degree adjusting mechanism, by narrowing the flow of material, the material stays in the kneading section and the degree of kneading becomes high.
In Patent Document 24 is described a kneading degree adjusting mechanism having a rotary valve. The rotary valve is provided with two discs each having a plurality of through holes spaced apart in the circumferential direction, the two discs being arranged axially side by side. That is, in this kneading degree adjusting mechanism, the through holes are opened or closed by displacing the discs circumferentially from each other. When the through holes are closed, the material stays in a kneading section and the degree of kneading becomes high.
In Patent Document 25 is described a kneading degree adjusting mechanism having a shielding wall for shutting off the flow of material. A resin channel is formed in the shielding wall so that the degree of opening thereof can be adjusted. That is, in this kneading degree adjusting mechanism, the degree of material kneading is adjusted by conducting the material into the resin channel formed in the shielding wall and then changing the degree of opening of the resin channel.
The above kneading degree adjusting mechanisms (1) to (3) involve the following problems.
(1) Problems of Gate Type Kneading Degree Adjusting Mechanism
In the gate type kneading degree adjusting mechanism, each gate member is cut out in conformity with either the outside diameter of a circular section part having a diameter smaller than the outside diameter of a kneading screw or the inside diameter of a barrel having almost equal diameter to a circular section part having equal diameter to the outside diameter of the kneading screw. When the gate member performs an opening motion, the shape of its cutout portion comes to be no longer in conformity with the inside diameter of the barrel or the outside diameter of the circular section part. Consequently, in either open or closed condition, the material stays in a stepped portion formed by a dent in the barrel inner wall or in a stepped portion formed by protrusion of the gate member, so that contamination is apt to occur.
In the gate type kneading degree adjusting mechanism, moreover, even in a fully closed condition of the gate members, a gap is formed between the kneading screw and the gate members to avoid a metal-to-metal contact between the kneading screw and the gate members. Therefore, it is difficult to let the narrowing effect be exhibited in the fully closed condition. In the gate type kneading degree adjusting mechanism there further arises the problem that a large space is needed for ensuring a gate opening/closing stroke to permit extraction of the kneading screw in maintenance.
Further, the gate type kneading degree adjusting mechanism involves the problem that at the beginning of gate opening it is impossible to adjust the kneading degree with a high accuracy. More particularly, the degree of kneading changes abruptly if the gate members are moved even slightly from the fully closed condition. Consequently, in the gate type kneading degree adjusting mechanism, the adjustment of narrowing becomes very difficult in an initial stage of opening of the gate members. This is known well to those skilled in the art.
(2) Problems of Slot Bar Type Kneading Degree Adjusting Mechanism
In the slot bar type kneading degree adjusting mechanism, each gate bar has both a cutout portion along the inner periphery surface of the barrel in a fully open condition of the gate bar and another cutout portion which approaches the circular section part of the kneading screw in a fully closed condition of the gate bar. Therefore, in the fully closed condition of the gate bars it is inevitable structurally that a large gap is formed between the gate bars and the outer periphery surface of the kneading screw, particularly between the gate bars and the kneading screw at right and left and central positions when looking at the cutout portions axially. Consequently, it is more difficult than the foregoing gate type to let the narrowing effect be exhibited.
In the slot bar type kneading degree adjusting mechanism, moreover, in both fully open and fully closed conditions, since the outer periphery surface of each gate bar is projected or depressed from the inner periphery surface of the barrel, the material to be kneaded is apt to stay in this portion. That is, in this kneading degree adjusting mechanism it is very likely that there will occur contamination due to the staying of the material to be kneaded, with consequent exertion of a bad influence on the product quality.
However, the most serious problem encountered in the slot bar type kneading degree adjusting mechanism is that the degree of kneading cannot be adjusted with a high accuracy, like the gate type kneading degree adjusting mechanism. That is, also in the slot bar type kneading degree adjusting mechanism, both arcuate portions, i.e., the gate bar cutout portion and the screw outer periphery surface, are moved closer to or apart from each other. Therefore, if the gate bars are rotated even slightly from the fully closed condition, the degree of kneading is apt to change abruptly. For this reason, adjustment of the degree of kneading, especially adjustment of narrowing in an initial stage of the opening motion of the gate bars, is very difficult.
(3) Problems of Other Types of Kneading Degree Adjusting Mechanisms
The kneading degree adjusting mechanisms of Patent Documents 20 to 25 each involve the problem that the conveyance volume of the screw, i.e., production capacity, is greatly limited because the reduced-diameter portion is provided and the problem that the operation for pulling out the screw from the barrel becomes complicated.